1. Field of Invention
The invention relates generally to instruments for examining and treating the eye and specifically to a scanning laser ophthalmoscope equipped with external laser sources for the purpose of retinal microphotocoagulation.
2. Description of Prior Art
The ophthalmoscope is well known as an important device for examining the eye, and in particular the retina. As a result of great interest in preserving eyesight, ophthalmoscopes of various constructions have been built. The latest version of the ophthalmoscope, a scanning laser ophthalmoscope, is particularly appealing because of its unique capability of combining infra-red and angiographical imaging of the retina with psychophysical procedures such as the study of visual fixation characteristics, visual acuity measurements, and microperimetry. A precise correlation between retinal anatomy and retinal functioning can be established with the scanning laser ophthalmoscope. This retinal function mapping is now known to be very helpful to the surgeon when applying therapeutic laser. Until now however, these therapeutic laser applications have been delivered to the retina with an instrument other than the scanning laser ophthalmoscope. The use of different instruments renders the comparison of images, the interpretation of psychophysical testing and precision of treatment more difficult.
U.S. Pat. No. 4,213,678, issued Sep. 29, 1980 to Pomerantzeff et al, discloses a co-pupillary scanning laser ophthalmoscope for the purpose of diagnosing and treating retinal disease using two different intensity levels of the scanning laser beam. One intensity range can be used for monochromatic imaging and angiography while a much higher level of the same laser beam or a different coaxial scanning laser beam is used for retinal photocoagulation. This novel approach however is not ideal because of the technical difficulties in implementing safety controls for such a scanning therapeutic laser beam, the difficulty in modulating the scanning laser beam over a range from non-coagulating to coagulating energies at video bandwidths, and the non-thermal complications of high intensity pulsed laser beams in the nanosecond domain with an inappropriate duty cycle. Pulsed thermal microphotocoagulation is however useful to restrict the impact of therapeutic applications to selective layers of the retina, specifically the retinal pigment epithelium. An appropriate duty cycle is necessary. The only solution that is left for effective thermal coagulation, wether pulsed or continuous, is to combine the scanning laser ophthalmoscope with a traditional external non-scanning therapeutic laser source. However, it is impossible to image directly the impact of such therapeutic laser source with a traditional co-pupillary scanning laser ophthalmoscope.
In the prior art, an ophthalmoscope, exemplified by the biomicroscope, is optically combined with a non-scanning therapeutic laser source for the purpose of retinal photocoagulation. In this modality, a contactglass is usually placed on the cornea to be able to view the retina with the instrument, and a mirror is used for reflecting the therapeutic laser beam onto the desired retinal location through a small part of the pupillary area. Importantly, the retina is illuminated and observed through different parts of the pupillary area to avoid reflexes, i.e. Gullstrand""s principle of ophthalmoscopy. This optical arrangement makes the art of precise focussing of a small therapeutic laser beam on specific retinal levels, especially a Gaussian beam more difficult in the presence of wavefront aberrations or large diameter entrance beams. More specifically, focussing with a contactglass on the retina is subject to eye and head movements of the patient that easily brings the Gaussian beam out of focus because of its high divergence.
Small therapeutic applications are often desired because they save retinal tissue, also they can be tailored to the shape of the lesion and they can take a variability in absorption more easily into account. However, photocoagulating ophthalmoscopes have been limited when consistent small or localized laser applications in the retina are desired because the anatomical changes caused by the therapeutic laser are difficult if not impossible to visualize during treatment in the presence of photocoagulating light. This is even more the case if minimal intensity, i.e. threshold applications are desired. The critical endpoint of the laser application is often exceeded. The surgeon, upon recognizing the minimal anatomical changes on the retina, is also handicapped by a substantial human reaction time delay before he can interrupt the therapeutic laser. During this delay the laser continues to deliver energy to the retina and changes in the subject""s fixation may occur. Since the reaction time of the surgeon may exceed 200 ms, a 100 ms laser application can easily be wrongly targeted on the retina in the case of misalignment.
Also, it is difficult to permanently document previous applications on the retinal image because threshold applications themselves are usually not visible some time after the initial treatment.
The principal object of this invention is to combine in one instrument the capabilities of imaging, psychophysics, and microphotocoagulation with optimal focussing and documenting of variable size therapeutic laser applications that are used in threshold and pulsed selective microphotocoagulation. This principal object is accomplished by selecting an entrance location of a spatially and temporally modulated external therapeutic laser beam that is subject to minimal wavefront aberrations through observation of the retina with the scanning laser ophthalmoscope using the same entrance location for the scanning lasers. As documented in the prior art, Gullstrand""s principle is used differently in scanning laser ophthalmoscopy, hence the possibility and necessity to use a similar optical pathway for both the therapeutic and scanning laser beams. To implement this solution, several principles have to be taken into account:
(1) A special coupling system between a confocal scanning laser ophthalmoscope and external laser sources is used. It comprises an optimized beamsplitter and dedicated opto-mechanical linkage device. This linkage device allows the alignment of the pivot point for the fast scanning diagnostic laser beams of the scanning laser ophthalmoscope with the pivot point of the non-scanning external therapeutic laser beams. Optimizing the Maxwellian viewing of a retinal location will then also result in a minimal wavefront aberration for the external laser beams because the same pivot point is used. Also in this situation, the amount of prefocussing necessary to image on a specific retinal layer is a reference, if needed, for focussing the therapeutic laser beam with its proper telescopic optics. This telescopic optics can also take into account minimal chromatic aberrations caused by differences in the wavelengths used. The main focusing mirror of the SLO can be incorporated into the opto-mechanical device to simplify the mechanical angulating mechanism of the external therapeutic laser beam.
(2) As mentioned before, a non-confocal or co-pupillary scanning laser ophthalmoscope cannot be used to detect the impact of the external laser beam on the retina. The confocal instrument can do this, however under specific conditions only. It is important to realize that the reflection image of the therapeutic application on the monitor is actually a convolution of the actual external laser spot with the confocal aperture. Usually the confocal aperture of the scanning laser ophthalmoscope is larger and hence the backscatter image cannot be used directly to determine size or adjust focussing. The foregoing necessitates either indirect focussing as mentioned before by using the same pivot point and relying on the focussing of the retinal image, or preferably the use of a specially constructed double aperture with different size pinholes.
(3) Although it is possible to realize part of the invention with one detector pathway, considerable advantages are derived from using two detectors that are temporally synchronized in the confocal scanning laser ophthalmoscope. Reasons are the weak contrast of the aiming beam on the retinal image, obscuring therapeutic light, and the fact that the retina and therapeutic laser spot can move independently of each other. Using an appropriate beamsplitter and filters, one detector images the retina, its pigment distribution and the anatomical changes caused by the therapeutic laser, unimpeded by the therapeutic laser light. Reference fiducial landmarks in the retinal image can be retrieved with two-dimensional normalized grayscale correlation faster than human reaction time would allow. A second synchronized detector images only the backscattered light from the external laser beams, without a background of moving retinal details. This image can be localized using simple image processing techniques such as look-up table manipulation. The implementation of a two detector pathway therefore allows registration of therapeutic laser applications, referenced on the retinal image, and the use of a safety shutter in case of excessive misaligment. It should be noted that this specific part of the invention could equally be applied to traditional photocoagulating systems if they are equipped with two video cameras, as long as the detector images are made spatially congruent.
(4) An aiming beam of different wavelength than the actual therapeutic laser source is polarized, and is transmitted after backscattering from the retina through the polarizing beamsplitter. Only the aiming beam wavelength, properly polarized, is allowed to reach a photodetector in order to avoid strong corneal reflections that may seem as a second spot on the retinal image.
(5) If the therapeutic laser beam is Gaussian in nature, it can easily be modified with the help of an acousto-optic modulator or two-dimensional acousto-optic deflector. This will create a pattern of microphotocoagulation spots of various duration and intensity that are easy to focus without a contactglass onto the retina with the SLO. Head movements of the patients become far less important as a near collimated and wide entrance beam is used on the cornea for which a large depth of focus or Raleigh zone exists. This cannot be accomplished with an ordinary slitlamp and eye contactglass. This is the major advantage of the current invention and continuation.
The invention allows to accurately place and document small, minimal intensity therapeutic laser applications to selected layers of the retina, hence the term microphotocoagulation or selective photocoagulation. With the proper selection of wavelength and therapeutic laser duration and pulse characteristics, selective targeting of retinal pigment epithelium layer and a various amount of sensory retina can be accomplished. Immediate microperimetric and angiographic feedback is available.
Microphotocoagulation has the ability to remove temporarily or permanently a percentage of the metabolically very active photoreceptors and retinal pigment epithelium cells, while minimizing damage to other anatomical structures, especially the nourishing choriocapillary layer, delicate Bruch""s membrane, ganglion cell layer and neural tissue in between applications. Virtual xe2x80x9coxygen windowsxe2x80x9d, reducing relative hypoxia, can for example be established through reduction of the demanding metabolic load of the central retina. This approach is useful in the retardation of onset or prevention of drusen related and neovascular age-related maculopathy. Possible mechanisms are an accelerated removal of material that thickens Bruch""s membrane and the reduced production of angiogenetic factors caused by relative hypoxia. Debridement of retinal pigment epithelial cells may lead to the removal of infectious agents, accumulated intracellular material or replacement of otherwise defective retinal pigment cells. The retinal location, focussing, size, intensity and duration of often invisible therapeutic laser applications can be stored and used for follow-up evaluation.
Further objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing description of a preferred embodiment.